/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.flink.runtime.io.disk.iomanager;

import org.apache.flink.runtime.io.network.buffer.Buffer;

import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

import static org.apache.flink.util.Preconditions.checkArgument;

/** Helper class to read {@link Buffer}s from files into objects. */
public class BufferFileChannelReader {
    private final ByteBuffer header = ByteBuffer.allocateDirect(8);
    private final FileChannel fileChannel;

    BufferFileChannelReader(FileChannel fileChannel) {
        this.fileChannel = fileChannel;
    }

    /**
     * Reads data from the object's file channel into the given buffer.
     *
     * @param buffer the buffer to read into
     * @return whether the end of the file has been reached (<tt>true</tt>) or not (<tt>false</tt>)
     */
    public boolean readBufferFromFileChannel(Buffer buffer) throws IOException {
        checkArgument(fileChannel.size() - fileChannel.position() > 0);

        // Read header
        header.clear();
        fileChannel.read(header);
        header.flip();

        final boolean isBuffer = header.getInt() == 1;
        final int size = header.getInt();

        if (size > buffer.getMaxCapacity()) {
            throw new IllegalStateException(
                    "Buffer is too small for data: "
                            + buffer.getMaxCapacity()
                            + " bytes available, but "
                            + size
                            + " needed. This is most likely due to an serialized event, which is larger than the buffer size.");
        }
        checkArgument(buffer.getSize() == 0, "Buffer not empty");

        fileChannel.read(buffer.getNioBuffer(0, size));
        buffer.setSize(size);
        buffer.setDataType(isBuffer ? Buffer.DataType.DATA_BUFFER : Buffer.DataType.EVENT_BUFFER);

        return fileChannel.size() - fileChannel.position() == 0;
    }
}
